Abstract
Disc springs are conical annular discs, which are characterized by a high spring force with a small spring travel and good space utilization. In operation, they must meet high demands on the stability of the spring characteristic and the fatigue strength. Under loading, tensile stresses occur which limit the possible applications of disc springs. Compressive stresses can be generated in the stressed areas by means of shot-peening in order to extend the operating limits for a given yield and fatigue strength. Since the spring geometry and characteristics change during shot-peening, the design of the shot-peening treatment is iterative and cumbersome. The present research proposes an incremental forming process for forming and integrated targeted adjustment of residual stresses in disc springs from metastable austenitic stainless steel (MASS), to achieve improved spring properties and high cyclic strength. The main mechanism of residual stress generation is the transformation of metastable austenite into martensite under the action of the forming tool. Different experimental characterization techniques like the hole drilling method, X-ray diffraction, disc compression tests, optical microscopy and cyclic tests are used to correlate the residual stresses and disc spring properties. A numerical model is developed for simulating the martensite transformation in disc springs manufacturing. The results prove that incremental forming enables process-integrated engineering of the desired compressive residual stresses, entailing a higher spring force of metastable austenitic disc springs in comparison to conventional disc springs. Due to martensite formation, the generated residual stresses are stable under cyclic loading, which is not the case for conventionally manufactured springs.
Highlights
Disc springs are conical discs, finding application in cases which require high spring forces in limited installation space
The second goal is to analyze the behavior of the residual stresses during cyclic loading, which in turn is dependent on the martensite stability, i.e., do the generated residual stresses along with deformation-induced martensite remain stable under cyclic loading or do they vanish with the increasing number of load cycles? The third goal of the current study is to develop a numerical model of the forming process of disc springs from metastable austenitic stainless steel (MASS) by incremental sheet forming (ISF)
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Summary
Disc springs are conical discs, finding application in cases which require high spring forces in limited installation space. Disc springs are expected to have a high fatigue strength with low stress relaxation [1]. The fatigue life and stress relaxation behavior of a disc spring are determined by its residual stress state and the service loads. Tensile residual stresses are present in the tensile-loaded underside of conventionally formed disc-springs. They can cause early fatigue failure of the components under service loading [2]. Compressive residual stresses increase the fatigue life by Materials 2019, 12, 1646; doi:10.3390/ma12101646 www.mdpi.com/journal/materials
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